Our objective is to determine why gastrin stimulates pepsin secretion from the intact and vagotomized stomach but not from Heidenhain pouches in dogs. We intend to look into the possibilities (a) that essential background parasympathetic activity is lacking in the pouch, (b) that there is unopposed and therefore enhanced pouch sympathetic activity, (c) that the vagotomized stomach has extravagal innervation not present in the Heidenhain pouch, (d) that Auerbach's plexus is necessary but non-functional in the pouch, and (e) that physical continuity between main stomach and pouch is needed. All of these experiments will be carried out in conscious dogs with gastric fistulae and either Heidenhain or Pavlov pouches. The latter will be subjected to nerve section and pedicle section in successive experiments. In the other experiments the tools will be mainly pharmacological, using specific autonomic stimulating and blocking agents. Pepsin will be estimated by Anson's hemoglobin method.